1. Field of the Invention
The present invention relates generally to the field of antennas and, more particularly, the present invention relates to improved radio telecommunication antennas.
2. Description of the Related Art
Information is communicated in radio communication systems through the transmission and reception of electromagnetic waves. Data to be communicated modulates an electromagnetic wave at a transmitter which produces signals that are radiated by an antenna. An antenna at a receiver detects the electromagnetic wave which is demodulated by the receiver, thereby reproducing the transmitted data. Electromagnetic waves containing data which are used for communication in this manner are known as radio signals. Antennas which are used in transmitting and receiving the electromagnetic signals provide gain to both transmitted and received radio signals.
In areas of radio communications such as satellite communications, the direction in which radio signals are transmitted and received is important. In satellite communications, radio signals must travel over large distances through a variety of unknown media. This results in a significant decrease in the power of the transmitted radio signal which ultimately reaches the receiver. In order to form an effective radio communication link, optimum use must be made of the transmitted and received radio signal power. For this reason, radio telecommunication systems often use antenna systems which maximize the gain of a radio signal transmitted in or received from a given direction.
An antenna system which is known in the art that operates to utilize the direction of transmission or reception of a radio signal is a planar phased array antenna. Such a planar phased array is comprised of a plurality of antennas arranged on a plane surface. Each antenna may transmit or receive a version of the radio signal. In the case of reception, for example, each antenna of the planar phased array delivers a version of the radio signal, although shifted in phase in accordance with the spatial separation of each antenna relative to the direction of arrival. A schematic diagram of such a planar phased array in which three antennas are incorporated is set forth in FIG. 1.
In FIG. 1, a planar phased array 1 is shown which is comprised of three antennas 2, 3, 4. Radio waves transmitted by a distant source (not shown) are received by the three antennas 2, 3, 4. Three versions of the received signal 5, 6, 7 are delivered by the three antennas 2, 3, 4. As a result of the spatial separation of the antennas, the received signals are displaced in time with respect to each other, resulting in the three versions of the received signal exhibiting a phase displacement in correspondence with the spatial separation of each antenna. Let r.sub.x (t) be the signal received by an antenna x at a time t, where, in this case x=1 to 3 for the three antennas 2, 3, 4. If the version of the signal received by the first antenna is given by equation (1), then the versions of the signal received by the second and third antennas 3, 4, will be those given by equations (2) and (3), where R.sub.x is the power in each version of the signal. Each version of the signal is then combined by a summer 8, to produce a resultant signal r(t). The combined received signal is represented by equation (4). ##EQU1##
As illustrated in FIG. 1, the planar phased array, is less suitable for detecting a radio signal 9, which has an angle of incidence which is greater than sixty degrees from the perpendicular 10. This will be discussed below.
A conventional planar phased array is mechanically steered to a desired direction in which the reception of a radio signal is optimized. The optimum direction of reception or transmission is that which causes the phase of each version of the signal to be the same. The phased array therefore operates so that the versions of the radio signal add constructively. As can be seen from the example in FIG. 1, this is achieved by steering the array until the axis upon which the antennas are mounted is perpendicular to the direction of propagation of the signal to be received. This causes the relative delays between each version of the received signal to be reduced to zero, resulting in no corresponding phase displacement. The signals therefore add constructively. Likewise a radio signal may be transmitted in a desired direction, by steering a corresponding phased array so that it points accordingly in a desired direction.
A known technique in which radio signals may be transmitted in, or received from a direction by an array of spatially displaced antennas, which does not involve mechanical movement of those antennas, is known as electronic beam steering. With this technique, the phase of each version of the radio signal is arranged to be shifted electronically, so that the versions of a radio signal add constructively for transmission or reception in a desired direction. The versions of the radio signal are therefore focussed into a beam pointing in the direction of transmission or reception. The direction in which the beam is focused is controlled electronically, providing means for the direction of focus to be dynamically adjusted.
Radio communication systems are designed to both transmit and receive information contemporaneously. An item of radio communications equipment which is provided with means for both transmission and reception of information is known within the art as a transceiver. The technique of contemporaneous transmission and reception is known as duplexing. Frequency division duplexing is a known duplexing technique in which the carrier frequency of transmitted and received radio signals is arranged to be different and separated by a suitable guard band of frequency. To separate the transmit signal from the received signal, a duplexing filter is required. The duplexing filter operates to prevent energy from the transmitted signal from corrupting the received signal. The duplexing filter must provide sufficient attenuation to a transmitted signal, so that little or no energy from the transmitted signal is present within the frequency band of the received signal.
One disadvantage of conventional planar phased arrays is that a separate duplexing filter is required for each antenna in the array. This increases both cost and size of the antenna system.
A further disadvantage of the planar phased array is that it is only suitable for steering a beam within a limited angle of incidence from a plane perpendicular to the axis in which the antennas are aligned. This is indicated in FIG. 1, where the planar phased array 1 is not suitable for beam steering by a radio signal 9 which has an angle of incidence greater than about sixty degrees from the perpendicular 10. To provide an antenna system with a hemispherical radio coverage pattern, for example, multiple planar phased arrays are required, making construction and testing of an antenna system difficult and further increasing its cost and size.
It is here stated that the term radio coverage where used herein means a volume which an antenna system is capable of illuminating with radio signals or from which an antenna system is capable of detecting radio signals with sufficient strength to effect radio communications.
It is an object of the present invention to provide an antenna system in which the aforementioned disadvantages of known antenna systems using planar phased arrays are eliminated.